swap nodes in pairs

# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class (object):
    def swapPairs(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next:
            return head
        
        cur = head
        next = head.next
        next_head = head.next.next
        
        next.next = cur
        cur.next = self.swapPairs(next_head)
        
        return next